There has been known a device disclosed in Patent Citation 1 as an engine starting device (hereinafter referred to as a starter) of a vehicle or the like. The starter is configured with a motor which generates a rotation force, a pinion gear which is disposed to be rotatable on an output shaft of the motor and to be movable in an axial direction, an electromagnetic switch which turns on and off the power of the motor, and the like. If the electromagnetic switch is operated (an electromagnetic force is generated to attract a plunger), the pinion gear is moved in the axial direction to be engaged with an engine starting gear such as a ring gear or the like of an engine (hereinafter, referred to as a “ring gear”), and thus, the ring gear is rotated and the engine is started.
In addition, at the time of the movement of the pinion gear in the axial direction, in a case where the teeth of the pinion gear collide with a side surface of the ring gear and cannot be engaged therewith, a reactive force is saved in a spring mounted in the electromagnet switch before closing a main contact of the motor. After that, if the main contact is closed to turn on the motor, the pinion gear is forcibly rotated with the rotation force of the motor, and at the time of rotation to a location where the engagement with the ring gear can be performed, the pinion gear is pressed in the axial direction by the reactive force saved in the spring to be engaged with the ring gear.
In a vehicle for improving recent environmental concerns, for objects of improvement of fuel efficiency and improvement of discharge emission, an idling stop system which controls automatic stop and restarting of the engine is gradually employed, and the number of usage of the starter tends to be increased. In addition, by assuming a case of receiving a restarting request at the time of a state where the engine rotation is not completely stopped in idling stop, a functional improvement is designed by engaging the pinion gear with the ring gear in the rotation of the ring gear to perform restarting. Accordingly, it is difficult to secure sufficient engagement durability in the starter of the related art.
There is known a method of processing a chamfering shape of the pinion gear in such a case, to prevent abrasion of the end surface of the tooth and to improve durability of a gear device without degrading the engagement property (for example, see Patent Citation 1). In addition, there is known a method of improving the engagement property by further processing the shape of the pinion gear (for example, see Patent Citation 2). Further, there is known a method of improving the durability by processing a structure of the pinion gear portion (for example, see Patent Citation 3).
[Patent Citation 1] JP-A-2001-248710 (Abstract, FIG. 3 and the description thereof)
[Patent Citation 2] JP-A-2009-168230 (Abstract, FIG. 1 and the description thereof)
[Patent Citation 3] JP-A-2006-161590 (Abstract, FIG. 2 and the description thereof)